Sintered sialon is known to be a chemically stable material, having high mechanical strength within a temperature range from room temperature to high temperature and high hardness which are superior to those of silicon nitride. Thus, sialon sintered bodies find a wide range of uses, such as machine parts, heat-resistant parts, and wear-resistant parts. One use of such a sialon sintered body is a cutting insert, which is attached to a cutting tool in use thereof (see, for example, Patent Documents 1 to 5). A cutting insert is a cutting edge to be removably attached to the tip of the main body of a cutting tool and serves as a tool part which is also called a throw-away chip, a cutting edge-replaceable chip, or the like.